1. Field Of The Invention
The present invention relates to dye diffusion thermal transfer printers.
2. Background Art
Dye diffusion thermal transfer involves the transport of a dye, or dyes, by the physical process of diffusion from a dye donor layer into a dye receiver layer. The highest rate of diffusion of the dye occurs when the glass transition temperature of the receiver layer is below that of the lowest temperature obtained during printing with the thermal head. Thus, high color densities are obtained under these conditions. In non-interactive dye diffusion, there is no chemical reaction between the dye and the receiver layer. These dyes are retained in the receiver matrix under ambient temperature conditions because the ambient temperatures are below the glass transition temperature T.sub.g of the dye receiver layer, and because diffusion is extremely slow below the glass transition temperature of the receiver layer.
Some known dyes chemically interact with the dye receiver matrix after being transferred by diffusion to the receiver layer from the dye donor layer. These are called "interactive" dyes, and they fall into several categories such as, for example: metallizable dyes as disclosed in U.S. Pat. No. 5,246,910; acid-base interaction dyes as disclosed in JP 05238174; dyes which can be protonated as disclosed in U.S. Pat. No. 4,880,769; and dyes capable of covalent bond formation as disclosed in U.S. Pat. No. 5,270,283.
Transfer of an interactive dye involves diffusion of a dye precursor into the receiver layer, followed by reaction of the dye with the receiver matrix to form a color. When interactive dyes react with the receiver matrix, the result is a strongly bound dye which does not depend on the glass transition temperature of the receiver layer for keeping properties. Receivers with low glass transition temperatures T.sub.g are used to expedite movement of the dye precursor from the receiver surface and into the receiver layer. Thus, higher dye transfer efficiencies can be obtained during the initial printing step with a lower energy input.
However, color formation in the dye receiver layer depends on a chemical reaction, and the color density may not fully develop if the thermal energy (the temperature attained or the time elapsed) is too low. Thus, color development is often augmented by a post-printing step such as thermal fusing. This practice adds extra time and cost to the printing process, and is therefore not desirable.
Accordingly, there has been a need for an interactive dye printing process that results in greater thermal energy transferred to the receiver than that obtained with conventional thermal heads alone so that interactive dyes can be caused to undergo more extensive reaction during the transfer step, yielding higher color densities.